Rotary piston pump

ABSTRACT

A rotary piston pump has at least two rotors ( 8 ) with associated drive shafts ( 26 ), wherein for each rotor ( 8 ) an associated bearing tube ( 28 ) is provided, which extends into the associated rotor ( 8 ) and through which the associated drive shaft ( 26 ) passes. Between the inside ( 24 ) of each rotor ( 8 ) and the outside of the bearing tube ( 28 ) a first gap ( 64 ) is provided. In operation, the first gap ( 64 ) contains cooling fluid and is part of a cooling fluid circuit.

This invention relates to a rotary piston pump, comprising at least two rotors with associated drive shafts, wherein for each rotor an associated bearing tube is provided, which extends into the associated rotor and through which the associated drive shaft passes, a first gap being provided between the inside of the rotor and the outside of the bearing tube.

This invention in particular relates to a dry-compression vacuum screw pump or a Roots pump of this kind.

In these vacuum pumps, a compressible medium having absolute pressures smaller than 1 mbar is compressed to atmospheric pressure, in which case the working space should be free from oil and wear.

The parallel drive shafts usually are synchronized with each other in a ratio of 1:1 by means of a transmission. The speed of the shafts either corresponds to that of the motor, or the motor speed is increased by an additional pair of spur gears. The contactless meshing counterrotating rotors form chambers which are transported from the suction side to the pressure side, thereby assuming a smaller volume, which is achieved by changing the pitch of the rotors.

The compression heat produced can be dissipated e.g. via the outside housing wall; it is also possible to cool the rotors from the inside, which involves, however, a considerable additional design effort. On the other hand, the thermal expansion of the parts should be minimized, which can only be achieved by means of cooling, so that smaller gaps between the rotors can be achieved, which in turn leads to a reduction of the gap leakage. Moreover, cooling can not only increase the efficiency, but the media, e.g. gases, which as a result of the compression would be brought to temperatures above 200° C. without cooling, can be delivered far below this temperature. Finally, lower temperatures also have an advantageous effect on the design and service life of the parts of the rotary piston pump.

As far as the mounting of the rotors is concerned, the generic WO 97/01038 discloses concepts which provide a so-called overhung mounting. The invention relates to a rotary piston pump with such overhung mounting of the rotors. The bearing tube of each rotor extends into an axial opening in the same. The bearing tube usually is stationarily mounted at one end, preferably by coupling to the pump housing. The associated drive shaft, which has a drive-side end and an end coupled to the rotor, then extends through the bearing tube.

The generic WO 97/01038 describes a complex cooling of the rotors, in which the bearing tubes themselves include cooling passages through which coolant flows. In addition, radiant heat is said to be transferred from the rotor to the bearing tube through a gap between rotor and bearing tube. Seal gas can also be introduced into the gap, which is provided for cooling and for protecting the bearing and drive region against the access of pumping medium or of substances contained in the pumping medium.

It is the object of the invention to provide a rotary piston pump, in particular a vacuum screw pump, of simple design, which in addition has a low-maintenance design.

In a rotary piston pump as mentioned above, this is achieved in that in operation the first gap contains cooling fluid, i.e. cooling fluid is transported through the same, and is part of a cooling fluid circuit. The first gap has a cooling fluid inlet and a spatially separate, i.e. physically separate, cooling fluid outlet. In the rotary piston pump of the invention, cooling via the first gap is much more effective, as cooling fluid is introduced into the gap, which allows a good dissipation of heat directly at the rotor. This cooling fluid is part of a cooling fluid circuit, so that cooler fluid is always supplied to the rotor. Due to the separate inlet and outlet, there is only one flow direction through the cooled gap portion, i.e. no dead spaces like a blind hole, into which cooling fluid must flow in and out again.

Therefore, the cooling passages provided in the wall of the bearing tube or in the drive shaft in accordance with the prior art can be omitted, which simplifies the manufacture and maintenance of the pump in accordance with the invention. Additional tubes, pockets, cavities and the like inside the rotor can also be omitted.

Preferably, between the respective inside of the bearing tube and the associated outside of the drive shaft a second gap is provided, which is part of the cooling fluid circuit. This means that the two gaps to be produced very easily are in flow connection with each other.

In this connection, the cooling fluid circuit preferably is designed such that cooling fluid first flows into the second, radially inner gap and then from the same into the first gap.

The bearing tube has a stationarily mounted end and an opposite free end extending into the rotor.

In accordance with one aspect, the invention provides that at the stationarily mounted end the second gap has an associated cooling fluid inlet and at the axially opposite end a cooling fluid outlet, which is in flow connection with a cooling fluid inlet of the first gap at the free end of the bearing tube. With an upright bearing tube, this means that the cooling fluid is pumped up in the second gap to the free end of the bearing tube, in order to be subsequently flung radially to the outside into the first gap and finally flow down along the inside of the rotor. Between the stationary bearing tube and the rotating rotor shear flows are produced, so that an optimum transfer of heat from the rotor to the fluid is ensured.

Between the end face of the free end of the respective bearing tube and the adjoining wall of the rotor, a connecting passage is provided between the two gaps.

The invention provides some more advantages. Whereas in the prior art, all bearings should be expensive, sealed, permanently lubricated bearings and expensive additional gaskets were desired, the invention goes the opposite way. It provides, for instance, that between the respective bearing tube and the associated drive shaft at least one bearing is mounted, through which the flow of cooling fluid passes completely or partially, so that the bearing is cooled and lubricated.

Optionally or additionally, this can also be effected in at least one bearing between the bearing tube and the associated rotor.

Between the bearing(s) and the adjoining parts one or more bypasses for cooling fluid can be provided. These bypasses increase the flow rate, or if the bearing should be sealed contrary to what has been explained above, it allows the passage of cooling fluid in the vicinity of the corresponding bearing point.

In terms of flow, the cooling fluid circuit preferably is open towards the cooling and lubricating fluid of the transmission.

A particular advantage of the invention consists in that the cooling and lubricating fluid of the transmission for driving the rotors also constitutes the cooling fluid of the rotors. The hermetic sealing provided in the prior art thus can be omitted, and the entire pump has a much simpler design. In other words: that part of the interior of the transmission, which is filled with cooling and lubricating fluid, constitutes part of the cooling fluid circuit.

The simple design of the pump in accordance with the invention is also revealed by the fact that a cooling fluid pump driven by the transmission itself is accommodated in the transmission housing, which pump delivers the cooling fluid into the gap(s).

In accordance with the invention, the drive shaft is designed without cooling passage at least in the vicinity of its drive-side end (transmission end), but in particular along its entire length, which reduces the production costs and increases the stability.

A reservoir for cooling fluid, e.g. the transmission housing with the cooling and lubricating fluid contained therein, is in flow connection with the first or second gap via a passage located outside the drive shaft.

In accordance with the preferred embodiment, the rotary piston pump in accordance with the invention is designed free from seal gas.

Further features and advantages of the invention can be taken from the following description and the accompanying drawings, to which reference is made. In the drawings:

FIG. 1 shows a longitudinal section through a first embodiment of the rotary piston pump of the invention, which is designed as vacuum screw pump;

FIG. 2 shows a longitudinal section through a second embodiment of the rotary piston pump of the invention, which is designed as vacuum screw pump;

FIG. 3 shows a longitudinal section through a bearing region of the rotors, which has been changed as compared to the preceding embodiments; and

FIG. 4 shows an enlarged view of the framed region X as shown in FIG. 3.

FIG. 1 shows a dry-compression rotary piston pump in the form of a vacuum screw pump, which on the vacuum side has a suction port 10 and on the pressure side a blow-off port 12, which are both connected with each other by a working space 14. In the working space 14, two parallel rotors 8 are accommodated, which have a helix 16 with a downwardly progressively decreasing pitch. The rotors 8 are meshing with each other, rotate in opposite directions and form chambers 18, which during the rotation of the rotors 8 are transported from the suction side to the pressure side, i.e. with a pump at rest from the top to the bottom, so that the pumping medium enclosed in the chambers is compressed towards the pressure side.

The two rotors 8 have a hollow interior, are mounted overhung, have the same geometry and the same structure inside also in terms of their bearing, so that for simplification purposes only the right-hand rotor 8 together with its bearing must be explained.

The rotor 8 has an axial through hole with an upper portion 20 of smaller diameter and an adjoining portion of larger diameter, which subsequently is defined by an inside 24. A drive shaft 26 is press-fit into the portion 20, so that rotor and drive shaft 26 are coupled to each other for joint rotation. Into the portion of the through hole of larger diameter, which is defined by the inside 24, a bearing tube 28 extends, which is stationarily mounted at a transmission housing 30, namely with its so-called lower, stationarily mounted end 31. Through this bearing tube 28, the drive shaft 26 extends into the interior 34 of the transmission housing 30. At the lower end, a spiral bevel pinion 38 is connected with the drive shaft 26, which pinion meshes with a spiral bevel gear 40 which in turn is firmly seated on a shaft 42, which is rotated by a non-illustrated motor. The two drive shafts 26 each have their own pair of spiral bevel pinions or gears 38, 40, but the spiral bevel gears 40 are mounted on a common shaft 42. The shaft 42 is in turn rotatably mounted in the transmission housing 30. The transmission arrangement is a so-called vertical shaft arrangement, in which the shaft 42 is perpendicular to the parallel drive shafts 26. By means of this arrangement, the speed of the drive shafts 26 can be increased (the pitch circle of the spiral bevel gears 40 is larger than that of the spiral bevel pinions 38), but at the same time the direction of rotation of the drive shafts 26 is synchronized.

The peripheral speed of the gears coupled to the drive shafts 26 is decisive for the transmission noise. By providing bevel gears, the peripheral speed was dependent on the axial distance in accordance with the prior art. In the pump in accordance with the invention, this is not the case; here, the peripheral speed of the spiral bevel gears 40 and the spiral bevel pinions 38 is independent of the axial distance, and the diameter of the spiral bevel pinions 38 even is distinctly smaller than the axial distance between the drive shafts 26. Another advantage of the design of the invention consists in that with the same gears different axial distances can be realized, when different rotors 8 are being used.

In the vicinity of the lower end, the drive shaft 26 is positioned in the bearing tube 28 via a locating bearing 50, which constitutes an open bearing, i.e. is not permanently lubricated and not sealed, and at the free upper end of the bearing tube 28 via a floating bearing 42 in the axial and radial directions. Hence, the rotor 8 is also supported in the axial and peripheral directions. Moreover, the bearing 42 neither is sealed, but constitutes an open bearing.

For cooling each rotor 8, each rotor has its own cooling fluid circuit, through which the cooling and lubricating fluid 60 in the interior of the transmission housing 30 is delivered, which is present for lubricating and cooling the gears provided therein. The cooling and lubricating fluid 60 in the transmission housing constitutes a reservoir for the cooling fluid of the rotors 8.

Thus, the cooling fluid circuit proceeds from the interior of the transmission housing 30 and extends through the open locating bearing 50 and/or a bypass 32 provided there, i.e. a passage provided outside the drive shaft 26. Between the drive shaft 26 and the bearing tube 28 a cylindrical annular gap is obtained, which extends up to the bearing 42. In the following, this gap 62 is referred to as radially inner, second gap. It is in flow connection with a first, radially outer gap 64, which is formed between the inside 24 of the rotor 8 and the outside of the bearing tube 28. The flow connection between the gap 62 and the gap 64 is effected via the open floating bearing 42, an optionally provided bypass 70 as well as groove-like connecting passages or an annular gap 80 between the end face of the free end of the bearing tube 28 and the adjoining, end-face wall of the rotor 8. This connecting passage 80 then leads to the cooling fluid inlet 81 (upper end) of the first gap 64. The cooling fluid outlet 83 of the first gap 64 is provided at its lower end, where a passage 90 leads into a collecting ring and from there into a non-illustrated oil sump or into the interior 34 of the transmission.

At the fluid inlet, the lower end of the gap 62, the cooling fluid thus flows into said gap, after having possibly cooled and lubricated the bearing 50, flows upwards to the fluid outlet towards the bearing 42 and/or the bypass 70, in order to then reach the gap 64 via the connecting passage 80, where it is pressed against the inside 24 of the rotor by the existing centrifugal forces and where shear flows are produced. The rotors 8, which are heated during compression, largely dissipate the heat to the cooling fluid, which then flows to the cooling fluid source, where it is mixed with the cold cooling fluid 60.

The illustrated pump is also characterized by a very simple sealing. On the vacuum side, no sealing is required at all. Seals 92 are only required on the pressure side of the vacuum pump between the lower end of the rotors 8 and the transmission. But since there is a connection there with the blow-off port 12 of the pump and hence with the atmosphere, the seals 92 never are acted on with pressure, which increases their service life and their sealing performance. Seal gas likewise can be omitted.

The embodiments as shown in FIGS. 2 to 4 substantially correspond to the one as shown in FIG. 1, so that subsequently only the differences will be discussed. It should be emphasized that these distinguishing features discussed below can also be combined with each other as desired within the scope of the illustrated embodiments.

In the embodiment as shown in FIG. 2, the left end of the shaft 42 is not mounted in the transmission housing 30, as here a shaft extension 100 is provided as drive for an integrated cooling fluid pump 110, which is accommodated in the interior 34 of the transmission and pumps the cooling fluid 60 to each of the second gaps 62. Corresponding conduits are designated with 120. Between the spiral bevel gears 40 there extends a rib 130 of the transmission housing 30, in which the shaft 42 is mounted in addition. A corresponding locating bearing is designated with 132.

The locating bearing 132 between the spiral bevel gears 40 is advantageous, because under the supply of heat the shaft 42 can freely expand towards both axial ends.

In the embodiment as shown in FIGS. 3 and 4, an open floating bearing 150 is once more provided between the inside 24 and the bearing tube 28 at the lower end of each rotor 8, by means of which bearing the corresponding rotor 8 is additionally stabilized at the lower end. The bearing 150 preferably is a plain bearing of relatively simple design, which is bypassed by a part of the cooling fluid through a bypass 160 in the form of a longitudinal groove in the bearing tube 28.

The cooling fluid preferably is oil.

As an alternative to the illustrated vacuum screw pump, the design with the cooling circuit in the gaps 62, 64 can also be provided in a Roots pump.

The pump of the invention is characterized by a very simple structure, by the lack of complex passages inside the rotor, the bearing tube and the drive shaft, and by very large surfaces, which serve a fast transfer of heat for dissipating the heat.

As shown, the housing 170, which surrounds the rotors 8, can of course also include an additional cooling passage 180 with cooling fluid. 

1. A rotary piston pump, comprising at least two rotors with associated drive shafts, an associated bearing tube for each rotor, said bearing tube extending into the associated rotor and through which the associated drive shaft (26) passes, and a first gap being provided between the inside of each rotor and the outside of the bearing tube, in operations, the first gap contains cooling fluid, is part of a cooling fluid circuit and has a cooling fluid inlet and a spatially separate cooling fluid outlet.
 2. The rotary piston pump as claimed in claim 1, wherein between the inside of the bearing tube and the outside of the drive shaft a second gap is provided, which is part of the cooling fluid circuit.
 3. The rotary piston pump as claimed in claim 2, wherein the cooling fluid circuit is formed such that cooling fluid first flows into the second gap and subsequently from the second gap into the first gap.
 4. The rotary piston pump as claimed in claim 3, wherein each bearing tube has a stationarily mounted end and a free end extending into the rotor, and, at the stationarily mounted end, the second gap has an associated cooling fluid inlet and, at the axially opposite end, the second gap has a cooling fluid outlet, which is in flow connection with a cooling fluid inlet of the first gap at the free end of the bearing tube.
 5. The rotary piston pump as claimed in claim 4, wherein between the end face of the free end of the respective bearing tube and the adjoining wall of the rotor at least one connecting passage is formed between the first and second gaps.
 6. The rotary piston pump as claimed in claim 1, wherein between the respective bearing tube and the associated drive shaft at least one bearing is provided, through which the flow of cooling fluid passes.
 7. The rotary piston pump as claimed in claim 1, wherein the respective bearing tube has a stationarily mounted end and a free end extending into the rotor, and in the vicinity of the free end a bearing is provided between the bearing tube and the associated drive shaft.
 8. The rotary piston pump as claimed in claim 1, wherein between the respective bearing tube and the associated rotor at least one bearing is arranged, through which the flow of cooling fluid passes.
 9. The rotary piston pump as claimed in claim 1, wherein the respective bearing tube has a stationarily mounted end and a free end extending into the rotor, and that in the vicinity of its own end associated with the stationarily mounted end the rotor is radially supported on the bearing tube by means of a bearing.
 10. The rotary piston pump as claimed in claim 1, wherein between the respective bearing tube and the associated drive shaft at least one bearing and an associated bypass is provided, so that cooling fluid can flow past the bearing through the bypass.
 11. The rotary piston pump as claimed in claim 1, wherein the rotors are coupled to each other via a transmission and the cooling fluid circuit is not sealed towards said transmission and towards the cooling and lubricating fluid contained therein for the transmission.
 12. The rotary piston pump as claimed in claim 11, wherein the rotors are coupled to each other via a transmission accommodated in a transmission housing and the cooling and lubricating fluid of the transmission flows into the cooling fluid circuit.
 13. The rotary piston pump as claimed in claim 1, wherein the rotors are coupled to each other via a transmission accommodated in a transmission housing and in the transmission housing a cooling fluid pump is accommodated, which is driven by the transmission and delivers the cooling fluid into the gap.
 14. The rotary piston pump as claimed in claim 1, wherein it is a vacuum screw pump or a Roots pump.
 15. The rotary piston pump as claimed in claim 6, wherein bearings for supporting the rotor on the bearing tube constitute open bearings.
 16. The rotary piston pump as claimed in claim 1, wherein the bearing tube is designed without cooling passage in its wall.
 17. The rotary piston pump as claimed in claim 1, wherein the drive shaft, at least in the vicinity of its drive-side end, constitutes a shaft without cooling passage.
 18. The rotary piston pump as claimed in claim 2, including a reservoir for cooling fluid, said cooling fluid reaching at least one of said gaps via a passage located outside the drive shaft.
 19. The rotary piston pump as claimed in claim 1, wherein the cooling fluid inlet and outlet of the first gap are provided in the vicinity of the opposite ends of the bearing tube.
 20. The rotary piston pump as claimed in claim 1, wherein it is free from seal gas.
 21. The rotary piston pump as claimed in claim 1, wherein between the respective bearing tube and the associated rotor at least one bearing and an associated bypass is provided, so that cooling fluid can flow past the bearing through the bypass.
 22. The rotary piston pump as claimed in claim 6, wherein bearings for supporting the rotor on the drive shaft in the bearing tube constitute open bearings.
 23. The rotary piston pump as claimed in claim 1, wherein the drive shaft, over its entire length, constitutes a shaft without cooling passage. 